What are the key skills that separates a junior from a mid level from a senior developer?

I'd say a junior dev knows a language or two, knows a framework/library/platform or two but has never built a full fledged thingamabob.

A midlevel dev has built a full fledged thingy (so they know the ins and outs of their language and framework a bit more) but hasn't had much experience maintaining it in production.

A senior dev can maintain the thingamabob in production, preferably more than one already and hopefully having junior to midlevel skills in other languages/frameworks.

Experience.

Experience, a junior Dev can know five languages or one

A mid level Dev has experience

A senior is the go to guy

It doesn't make a difference how many languages/frameworks you know if you are not proficient in any of them there's no point

Age! 😱
Junior: below 30
Midlvl: prior to midlife crisis
Senior: below 100 has passed m.c. already

Thats why there a 60y old midlvl: still not passed the crisis at all 😂

A medior developer has ever so slightly more experience than you, so forget about that promotion.

Go for manager.... Little to no experience, makes more money and gets bonuses for all developers hard work.

Senior dev is the one that creates things to make the work of the other two easier.

Experience and knowledge.

I'd argue a Senior differentiates himself/herself with maturity and their ability to manage technical with non-technical stakeholders.

I've seen many excellent developers who were just lacking in maturity and didn't behave in ways that befit a Senior position. If you want to know how a Senior behaves look at how existing Seniors behave for a benchmark

A senior developer understands and contributes to the full software lifecycle.

A junior is only concerned with their specific assigned tasks.

The intermediates are somewhere in the middle.

disiplin

Judging by these comments I should be a senior. Don't think my boss will agree...